This invention relates to the field of extraction, mixing and dispensing of solid products stored in bulk in a hopper or similar container. In particular, the invention relates to an apparatus which enables solid materials stored in bulk and which have poor flow characteristics to be extracted, mixed and dispensed.
Many solid materials are kept or held temporarily in hoppers, storage tanks and similar containers before being subjected to subsequent operations such as extraction, mixing, and dispensing with a view in particular to physical and/or chemical treatment. However, certain bulk materials experience a great deal of difficulty in flowing freely and totally by gravity from the containers in which they are stored; this is the case, for example, of powdered materials of relatively low specific gravity, fibrous materials with various levels of moisture content, granulate and aggregate materials of various type forms, products made of materials having a high friction coefficient and materials which form lumps or clumps spontaneously, for example when subjected to atmospheric humidity. Of these products, the following may be cited for illustrative purposes: large and small wood chips or fiber which is to be used for papermaking and in the particle-board industry; unprocessed or defibered asbestos, particularly compacted asbestos; scrap from seals made of asbestos-rubber, similar or equivalent material; scraps and pieces of cardboard and paper confetti of various sizes; and defibered cellulose in sheet or flake form; granulated rubber; certain minerals or chemicals that are frequently moist. In this last category are: sand, potash, urea and the like; fine fragments arising from cutting plastic film scrap; expanded and granular materials; atomized silica; pieces of scrap materials such as leather, fiberglass, and the like; and fiber-dusting products and similar products and materials.
Extraction of the aforementioned materials and products having poor flow characteristics from their storage containers poses a series of problems difficult to resolve. Indeed, whatever the shape of the bin, hopper or container, gaps often occur along the axis or axes of the discharge opening or openings, and the product or material forms into bridges from the base or at any other level of its container. When the container is full or nearly full, the flow of material is initially normal through the discharge opening or openings situated at the base of the container which may be equipped with an Archimedes' screw conveyor or the like. Rather rapidly, however, a bridge of material forms or a void appears when the layers of material in the vicinity of the walls of the container stop sliding downwardly or a gap forms above the discharge opening or openings; these two phenomena can coexist. Thus, although the flow may be normal at the beginning, the flow becomes irregular and often ceases altogether, while the container still contains a large volume of bulk material to be discharged.
Various apparatus have been proposed in attempts to remedy the above-mentioned difficulties and shortcomings. For example, the bottom of the container can be provided with a material agitator having scrapers or blades of various shapes or with revolving cutters. These known apparatus are satisfactory for materials of relatively low specific gravity and materials for which the bridging phenomenon is not very marked. However, apparatus of this type are ineffective in the case of dense products whose form is such that a tangling phenomena can develop, giving rise to very solid bridges and/or vaults, or in the case of moist or dry products, which tend to form clumps or lumps. Accordingly to one known improvement, agitators can be provided whose blades are located at various levels in the tank, bin or other container. In this way efficiency is often improved, but the costs of construction and maintenance are high and such devices require substantial drive power when the stored product has a high or even average density. In other known apparatus the phenomenon of vibration is employed by installing one or more vibrators on the walls of the tank or container or by providing a vibrating cone at its base, in which case the product flows along the periphery situated between this cone and an opening in the container. These known apparatus have some effectiveness, particularly in the case of finely divided, granular powdered products, but they do not enable products or materials with a fibrous structure or materials that tend to lump or to clump to be effectively extracted from the container. In addition, the vibrations can, in many instances, increase the difficulty of extraction because the vibrations tend to cause the materials in the container to heap and to compact. It has also been proposed that one or more material-extracting screw conveyors be installed in the container, for example a series of Archimedes' screw conveyors arranged in parallel side-by-side over the entire bottom of the container. In this case the initial outflow is accelerated but there is no apprecaable effect on the bridging phenomeon itself, because the screw conveyors extract only the material located close to them and not the material a little further away.